One way of manufacturing an optical preform is the plasma chemical vapour deposition (PCVD) process, which is known from U.S. Pat. No. 4,314,833 granted in the name of the present applicant. According to the process that is known therefrom, one or more doped or undoped vitreous layers are coated onto the interior of a substrate tube, using a low-pressure plasma in the glass substrate tube. After the vitreous layers have been coated onto the interior of the glass substrate tube, the glass substrate tube is subsequently collapsed into a solid rod through the application of heat. In a special embodiment, an additional amount of glass may be externally coated onto the massive rod, for example by means of an external vapour deposition process or by using one or more preformed glass tubes, providing a composite preform. From the preform thus obtained, optical fibres are obtained by heating one end thereof.
According to International application WO 99/35304 filed in the name of the present applicants, microwaves from a microwave generator are carried to an applicator via a waveguide, which applicator surrounds the glass substrate tube. The applicator causes high-frequency energy to be coupled into the plasma. The reactive gases, which may or may not be doped, are supplied to one side of the substrate tube, after which a reaction takes place under the influence of the plasma and doped or undoped vitreous layers are deposited on the interior of the substrate tube. The other side of the substrate tube is connected to a vacuum pump, so that a reduced pressure, generally a pressure ranging from 5 to 50 mbar, is generated in the substrate tube. The applicator is moved back and forth in the longitudinal direction of the substrate tube, and a thin vitreous layer is deposited on the interior of the substrate tube with every stroke. The applicator and the substrate tube are generally surrounded by a furnace so as to maintain the substrate tube at a temperature of 900-1300° C. during the deposition process.